Color picture tube screen



u ,1960 uni-51L 2,947,898 I -COLOR PICYTURE wuss SCREEN Filed March 16,1956 2 Sheets-Sheet 1 CENTERS OF DEFLECTION INVE NTORI v HANS HEIL, v 'iBYWW W ms AT RNEY Aug. 2, 1960 H. HElL 001.012 PICTURE TUBE SCREEN FiledMarch 16, 1956 2 Sheets-Sheet 2 FIGJO.

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M m F INVENTOR: HANS HEIL,

BY @Z LM HIS .A RNE ice- COLOR PICTURE TUBE SCREEN Hans Heil, Syracuse,N.Y., assignor to General Electric Company, a corporation of New YorkFiled Mar. 16,1956, Ser. No. 572,005

6 Claims. (Cl. 313-92) This invention relates to improvements inmultiplephosphor mosaic screens for cathode ray tubes of the so-calledpost acceleration type.

In cathode ray tubes of the post acceleration type the electron beam iscaused to pass through an accelerating field formed between the screenand an electron permeable grid or lens mask spaced therefrom, and thefield changes the velocity of the beam to a desired level before impactwith the screen. One advantageous form of screen for such tubes consistsof a plurality of groups of line-like or dot-like areas of sub-elementalimage dimension in at least one sweep coordinate of the electron beam orbeams, the areas in each group consisting of a plurality of respectivedilferent color light emitting phosphors. The lens mask cooperating withsuch a screen has a plurality of openings or electron permeable areas sopositioned with respect to the phosphor groups as to provide selectiveillumination of the different respective phosphor areas in each group bythe electron beams from ditferent respective electron guns. In one formof triphosphor color television picture tube, for example, each phosphorgroup of the mosaic screen consists of a set or triad of three parallelred, green, and blue phosphor lines, the lines being oriented parallelto the vertical sweep coordinate and having a width, or dimension asmeasured in the horizontal sweep coordinate, equal to a sub-elementalpicture dimension. The lens mask consists of a grille of paralleluniformly spaced wires, with the wires arranged vertically and spaced sothat there is one inter-wire spacing for each triad of lines. The grilleis so positioned relative to the screen as to permit the beam from eachof three electron guns to strike only the lines of the one colorphosphor for which it is intended.

The post acceleration field in such cathode ray tubes causes adistortion or curvature of the electron beam trajectory in the spacebetween the mask and screen, and thereby makes the pattern of thelanding points of any one electron beam on the screen different from asimple Crookes shadow of the grid. This distortion increases withdeflection angle, and for any one electron beam is rotationallysymmetrical about a center of distortion located at the point at whichthe beam strikes the screen when undeflected, i.e. when its deflectionangle is zero. In tubes employing a plurality of electron beams whosepaths are spaced before deflection, this distortion causes a compressionor reduction in the spacing of the landing points of the beams whichincreases with deflection angle. In other words the separation ofadjacent beam landing points in each phosphor group becomes smaller, andthe gaps between landing points of beams in adjacent phosphor groupsbecome larger, as the sides of the picture area are approached. Thiscompression is suflicient to be significant in tubes of televisionpicture size and must be taken into account when laying down thephosphor increments of the screen in order to avoid beam-phosphormisregistry and consequent color impurity'during tube operation.

Accordingly, a principal object of the present inven tion is to providean improved method for laying down the phosphorareas of a mosaicmultiple-phosphor screen for a post acceleration cathode ray tube whichproperly locates the individual phosphor areas in the direction of theirsub-elemental image dimension so as to insure proper registry thereofwith the scanning electron beams.

Another object is to provide an improved multiplephosphor screen andmethod of making thereof for a post acceleration cathode ray tube whichprovides maximum tolerances for shifting of the electron beam landingpoints without causing beam-phosphor misregistry or color impurity.

These and other objects of my invention will be better understood fromthe description hereinafter, and the scope of the invention will bedefined in the appended claims.

Briefly, the foregoing objects are accomplished according to a preferredaspect of the present invention by tailoring the sub-elemental imagedimension and adjusting the location of the phosphor areas in eachphosphor group in at least the marginal portions of the screen so thatsuch phosphor areas coincide in location and spacing with the electronbeam landing points thereon.

That is, on the portion of the screen where the electron beam deflectionangles are largest and the beam landing points are correspondingly mostclosely spaced, the subelemental image dimension of the individualphosphor areas is correspondingly reduced to provide a match betweenphosphor area size and beam landing point spacing which enable theelectron beams to land exactly cen- 'ter'ed on the respective phosphorareas. In another pre ferred aspect of the invention the sub-elementaldimension of the phosphor areas is progressively decreased from thecenter to the edge of the screen in exact correspondence with theprogressive decrease in beam landing eration cathode ray tube to whichthe present invention may be applied;

Figure 2 is an enlarged diagrammatic view showing I in an exaggeratedfashion the geometrical relations of a portion of the tube of Fig. 11;

Figure 3 is a further enlarged diagrammatic fragmentary view of aportion of the tube of Figure 1;

Figure 4 is a diagrammatic view showing illustrative electron beamlanding point positions on the screen of a cathode ray tube;

Figure 5 is a view similar to Figure 4 showing other beam landing pointpositions;

Figure 6 is a schematic view of a photographic arrangement suitable foruse with the present invention;

Figure 7 is an enlarged fragmentary perspective view of one type ofscreen pattern transfer arrangement suitable for use in the presentinvention;

Figure 8 is a view similar to Fig. 7, showing a later stage in thetransfer process;

Figure 9 is an enlarged fragmentary view showing one manner ofregistration of the electron beams of the tube with certain of thephosphor elements in the central portion of the screen;

Figure 10 is a view similar to Figure 9 illustrating the manner ofelectron beam-phosphor registry. in another portion of the screen;

manner of registry of electron beams and phosphor elements;

25,947,898 Patented Aug; 2., 19 50 Figure 13 is a view similar to Figure12 showing beam-phosphor registry in another portion of the screen;

Figure 14 is a view similar to Figure 9 showing still another manner ofbeam-phosphor registry; and

Figure 15 is a view similar to Figure 14 showingregistry in anotherportion of the tube.

Turning now to Figure l of the drawings, there is shown a postacceleration color television cathode ray picture tube of a type towhich the present invention is particularly applicable. The tubeincludes an envelope 2 closed at its front end by a substantiallycylindrical faceplate 4 on the inside surface of which is formed ascreen 6 including a plurality of groups of discrete phosphor areas 7,the areas in each group consisting of different color light emittingphosphors. For the sake of economy of description and ease ofunderstanding, the phosphor areas will be described specificallyhereinafter as line-like in shape and as having sub-elemental imagedimensions, i.e. width, in only one sweep coordinate. It will beappreciated by those skilled in the art, however, that the inventionembraces the formation of screens whose phosphor areas are eitherline-like or dot-like in shape, i.e. which have sub-elemental imagedimensions in either one or two sweep cordinates. Each phosphor linegroup includes one line-shaped area of each of three primary coloremitting phosphors, e.g. red, R, green, G, and blue, B, each line beingdeposited substantially parallel to the axis of the faceplate and havinga width of the order of a sub-elemental picture dimension. In onepractical embodiment of the tube, for example, of a size popularlyreferred to as 21 inch, the lines are approximately .010 inch in width.The tube also includes three electron guns 8 arranged side by side in aplane perpendicular to the phosphor lines, with the central gun beingpositioned so that its undeflected electron beam strikes the screen 6 atthe approximate center thereof. A deflection yoke 10 provides angulardeflection of the electron beams sufficient to sweep the screen bothhorizontally and vertically. The yoke is so oriented that the directionof vertical deflection is substantially parallel to the phosphor lines,while horizontal deflection is substantially perpendicular to thephosphor lines. The electron guns 8 are so spaced and arranged that thecenters of the deflection of the beams (center of deflection is thepoint defined by the intersection of the path of a beam before and afterdeflection) are equally spaced and lie in a common plane, the plane ofdeflection, which is substantially normal to the axis of the centerbeam. Spaced from the screen 6 is a substantially concentric electronpermeable mask or grid 12, here shown as a grille of fine wires 14arranged parallel to the phosphor lines and uniformly spaced to provideone inter-wire space opposite each phosphor line group. The screen ismaintained by a suitable power supply (not shown) at a potential Vsubstantially above the grille potential V so that there is asubstantial accelerating electric field between the grille and screen.The grille wires are so spaced and positioned with respect to the centerof deflection of each beam that the beam from each gun illuminates onlythe phosphor stripes, of the particular primary color for which it isintended.

Figure 2 shows diagrammatically the geometry of the tube of Figure 1.The surface of the screen is represented by line 30 while 32 denotes theplane of the grid, 34 designates the center of curvature of the screenand grid, point 36 the center of deflection of the center electron beam,and line 38 the trajectory of the undeflected center electron beam'from'its center of deflection to the screen. Point 40 represents thepoint at which the center electron beam passes through the grid afterdeflection in the horizontal sweep coordinate suflicient to give thebeam an angle of incidence a and point4'2 is the point at which theelectron beam passing through point 49 strikes the screen. The path ofthe electron beam from 40 to 42 is curved toward the center of the .4screen due to the eifect of the post acceleration electric field betweenthe grid and screen. Thus, the actual landing point of the electron beamon the screen is displaced toward the center of the screen from thepoint 44 at which the screen is intersected by the straight lineprojection of the path of the beam before reaching the grid. Thisdisplacement increases with increasing deflection angle. The term Rrepresents the distance between the point at which the undeflectedcenter electron beam strikes the screen and the point 46 at which thescreen is intersected by a radius '48 drawn from the center of curvature34 through point 40. The term r represents the distance between theactual landing point of a beam on the screen and the intersection withthe screen of a radius through the point at which the beam passesthrough the grid. Due to the increase in displacement of the beam by theaccelerating field with increased deflection angle, the dimension rincreases with increasing deflection angle, but ata decreasing rate.

As a result of this phenomenon, the three beams are effectively bunchedor compressed, i.e. the spacing between their landing points isdecreased, as deflection angle increases. This is best illustrated inFigure 3 wherein is shown in exaggerated fashion the deflection of allthree beams in the horizontal sweep coordinate, the points 50 and 52representing the centers of deflection of the left and right side beams,respectively. As may be seen from Figure 3, the spacing of the landingpoints of the center and two side beams at the center of the screen iswhere For tubes of television picture size it has been found that as apractical matter this decrease in spacing of the beam landing points maybe as much as 20% at maximum deflection angles.

Since the spacing of the landing points of the separate electron beamsis decreased toward the edges of the screen as shown in Figure 3,screens whose phosphor area sub-elemental dimensions are equal from thecenter to the edge of the screen will be impinged by the electron beamsin such a manner that when the lines are located r and dimensioned so asto be impinged at their centers by the undefiected beams, i.e. at thecenter of the screen as shown diagrammatically in Figure 4, the relativeposition of the beam landing points on lines of the same width at theedge of the screen will be as shown in Figure 5. The situation shown inFigure 5 is highly undesirable because when one of the beams such as thecenter beam shown is centered on its phosphor stripe the other two beamswill land very near the inner edges of their respective phosphorstripes. Hence, only a slight displacement of the landing points of thetwo side beams is required to shift them over to. the wrong color stripeand cause objectionable color impurity. Or, to put it another way, veryclose tolerances must be imposed on the beam landing point positionsinthe situation of Figure 5 in order to avoid color impurity.

According to the invention the phosphor areas of the screen are arrangedand located so as to coincide with 1 ge mane the above describeddistortions in the scan patterns of the electron beams and therebyinsure substantially exact.

or more properly of the correctlocationfor one set oflines to insureexact registry with a, trace of one electron beam. This master pattern'may'be formed in various ways, for example, by taking a'shadowphotograph of a mock grid 50, as shown in Figure 6, which is positionedrelative to the photographic plate 52, anilluminating light source 54,and a mask having a small aperture or slit 56 in a manner to simulatethe relative positions of the screen, mask and electron beam deflectioncenter in the actual tube. With this arrangement. a suitable opticalcorrector 58 is required to provide the necessary light ray distortionfor simulating electron beam curvature in the post acceleration field.Such apparatus is described more fully in the co-pending application ofH. Heil, Serial No. 555,368, filed December 27, 1955, and assigned tothe assignee of this application. The mock grid 50 may take any desiredform such as a sheet of opaque material having light transmitting slitstherein, a photographic plate having alternate opaque and transparentareas, or a transparent plate one side of which is coated with an opaquecoating into which is scratched transparent lines. Another way offorming the master pattern is by direct electron beam exposure of asheet of suitably electron sensitive material, such as a photographicemulsion, through an actual grid and under conditions simulating thosein regular tube operation.

Once the master pattern of one set of lines is formed, an image of thismaster pattern is then transferred to or impressed upon the screensupport member of the tube which in the tube shown is the faceplate 4.The image is positioned on the screen support member so that its centerof distortion coincides with the center of distortion of the trace ofone of the electron beams in the tube, e.g. the redphosphor-illuminating beam. In this way the image transferred to thescreen support member forms indicia of the positions. of one set of thephosphor lines, e.g. those to be illuminated by the red beam. Transferof the image of the master pattern to the screen support member may beaccomplished in various ways. For

example, the transfer may be made by making a photographic projectionprint or contact print of the master pattern on a photo-resist coatingon the screen support member which is light sensitive and also capableof serving as an adhesive or mechanical binder facilitating applicationof phosphor. One such known photo-resist material is polyvinyl alcoholsuitably light sensitized with ammonium dichromate. With this techniquethe indicia of the positions of the set of phosphor lines are formed inthe photo-resist coating by means of relatively insoluble areas spacedby relatively soluble areas which are dissolved away. The phosphoritself may then be applied to the insoluble areas of the resist and willbe retained thereby in proper location during completion of themanufacture of the screen. Other ways of transferring the image to thescreen support member .are by offset printing with a printing plate madefrom the master pattern or by stenciling with a stencil 61, as shown inFigures 7 and 8, made from the masterpattern, the printing ink in suchcases being, for example, a suitable pressure sensitive adhesive towhich the phosphor may subsequently be applied and which may later beconveniently removed as for example by evaporation during bake-out.

Still another way of forming indicia of the positions of a set ofphosphor lines on the screen support member is to coat the screensupport member with a photo-resist coating such as light sensitizedpolyvinyl alcohol, and then directly record on the coated screen supportmember itself a shadow photograph of a mock grid 50, in th manner of theshadow photograph technique above described and shown in Figure 6, usinga light source and suitable optical corrector as taught in the Heilapplication Serial No. 555,368 previously mentioned.

Additional images of the master pattern are then transferred to thescreen support member, to provide thereon indicia of each additional setof phosphor lines, e.g. blue and green. In accordance with theinvention, however,

each of these additional images is positioned so that its center ofdistortion coincides withthe, center of distortion of the trace of thecorresponding electron beam. That is, the indicia of the green lines areso placed'that their center of distortion coincides with the center ofdistortion of the trace of the green phosphor-illuminating electronbeam, and the indicia of the blue lines are placed with their center ofdistortion coinciding with the center of distortion of the trace of theblue beam. Since. thecenters of distortion of the several electron beam.

traces are spaced a distance equal to the spacing of the centers of thedeflection of the beams, which by definition isthe color base-S, theindicia for the successive sets of phosphor lines will likewise besuccessively displaced a distance of approximately S on the screensupport member. For proper color selection the exact spacing of the,

successive indicia must also, however, be a multiple of the individualphosphor line with s, preferably a multiple of the form [BN+(B1)]where'B is the number of different color phosphors employed and N is aninteger. Thus in the present case B is 3 and the exact displacement ofsuccessive indicia, which we may term S, should equal that value of theproduct (3N+2) (s) which most closely approaches S. a

The manner in which the relative displacement of the. indicia of theseveral sets of lines may best be accom-v plished is dependent upon theparticular method employed to transfer the images of the master patternto the screen support member If photographic contact printing from alight or electron beam exposed photographic master is employed, forexample, all that is required to shift the indicia is that the positionof the photographic negative be shifted relative to the screen supportmember a distance of S between successive transfers. If. oifset printingor stenciling 'is employed, as shown in Figures 7 and 8, the printingplate or stencil 61 should likewise be shifted S between successiveimage transfers. Thus by the above described form of the invention allthree sets of phosphor lines may be located on the screen support memberwith the use of but a single master pattern, and each set of lines willbe properly positioned for exact registry with its respectiveilluminating electron beam. Once the indicia for a given set of lines isformed the phosphor itself maybe applied thereto in any desired manner,as for example by application to the insolubilized portions of.polyvinyl alcohol resist or to the pressure,

thus effectively combining phosphor line location and ap plication.

In carrying out the foregoing method provision should preferably be madeto reduce the sub-elemental image dimension, i.e. width, of the phosphorlines from the.

center to the edges of the screen as necessary to conform to the reducedspacing of the electron beam landing points as illustrated in Figure 3,in order to avoid line overlap when the phosphor is applied to thescreen support member. Such line narrowing may be accomplished in avariety, of ways depending onthe-particular method used to transfer theimage of the master line pattern to the screen support member. Forexample, if-. a stencil 58 or printing plate is used to form the indiciaon the screen support member, the openings in the stencil or the contactareas of the printing plate may be progressively narrowed from thecenter to the edge so as to conform to the progressively reduced spacingof the beam landing points from the center to the edge of the picturearea. If the image transfer is accomplished. photographically, the linewidth may be reduced by known photographic means as for example byinserting avariable density filter 59, as shown in Figure 6, between theexposing light source and the photographic plate so as to reduce thelight intensity and hence progressively narrow the exposed areas of thephotographic emulsion from the center to the edge of the picture area.

The principal advantage of the above described screen printing method isthat the phosphor lines are so placed on the screen support member as toregister with the electron beam landing points in the central portion ofthe screen in the manner shown in Figure 9, where reference numerals 60,62, and 64 designate the red, green and blue beams, respectively and 66,68, 70' designate the red, green, and blue lines of one triad. At theside or edge portions of the screen, the lines will be narrower, but theelectron beams will still center on the lines in the manner shown inFigure 10, while at intermediate points between the center and edge ofthe picture area, beam-phosphor registry will be as shown in Figure 11.Thus, it may be seen that accurate registration of the beams with thecenter of the sub-elemental image dimen sion of the phosphor areasthroughout the entire screen.

is achieved by the technique above described. The invention thusprovides maximum tolerances for any shift in beam landing pointlocations which might be caused by various unforeseen factors such asfringing of the post acceleration field or other nonhomogeneous efiectswithout producing any color impurity.

In accordance with another form of the invention, conformance of thephosphor lines to the beam traces is accomplished by making the lines ofuniform width throughout the picture area, and increasing the actualcolor base in the tube to a value, S which is larger than the value Sfrom which the dimension s is derived. Increasing the actual color baseof the tube from S to S, increases the spacing of the beam landingpoints all across the screen and preferably S is made sufiiciently largeso that the beam landing points are centered on the phosphor lines atthe edges of the picture area as shown in Figure 12. The increasedtolerances thus obtained in beam landing point position at the edgepor-' Optionally, the value S may be chosen so that the beam landingpoint spacing equals the line width s, and the beams will thereforecenter on the lines, at an intermediate point between the center andedge of the.

screen. The width of the lines may thus be said to be a compromisebetween the spacing of the beam landing points at the center of thepicture area and at the edge of the picture area. When this is the casethe beamphosphor registry will be as shown in Figure M at the center ofthe screen and as shown in Figure at the edge of the screen.

Lines of uniform width s may be conveniently laid down by simplydisplacing the successive images repre, senting the successive indiciaof the respective 'sets of. phosphor lines a distance s on the screensupport member;

The manner in which this displacement may be achieved will depend uponthe particular method. employed to transfer the image of the masterpattern. to the screen support member and can be identical with any oneof the methods hereinbefore described for shifting the successive imagesby the distance S.

Thus there has been shown and described an improved multiple phosphormosaic screen and method of forming the same wherein due account istaken of the distortion of an electron beam scan pattern due toinfluences of a post acceleration field, and wherein provision is madeto locate each sub-elemental portion of the screen in a proper positionto insure optimum color purity and maximum tolerances in electron beamlanding point positions.

the invention may be carried out in various ways and may take variousforms and embodiments other than those illustrative embodimentsheretofore described. It is to be understood that the scope of theinvention is not limited by the details of the foregoing description,but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. 'In a cathode ray tube, a screen having two coor-- dinates, aplurality of electron guns arranged to illuminate the screen withelectrons, an electron permeable mask between the guns and screenadapted to have a potential difference with respect to the screen. suchas to form an accelerating field in the path of said electrons, and aplurality of groups of phosphors on said screen, each phosphor groupconsisting of respective phosphor areas adapted to be illuminated byelectrons.

from the respective guns, the phosphor areas in each group having adimension and a center to center spacing decreasing with increasingdeflection from the center of the screen in at least said one screencoordinate so as to correspond to the spacing in said one screencoordinate of the landing points of electrons thereon.

2. In a cathode ray tube, a screen, a plurality of electron gunsarranged to illuminate the screen with a plurality of electron beamsadapted to be swept across thescreen in two coordinates, a singleelectron permeable mask between the guns and screen adapted to have apotential difference with respect to the screen such as to form a singleaccelerating field therebetween, and a plurality of contiguous groups ofphosphors on said screen, each phosphor group consisting of respectivephosphor areas adapted to be illuminated by the respective electronbeams, the phosphor areas of said phosphor groups being contiguous andhaving an equal dimension in at least one sweep coordinate throughoutthe screen, approximately equal to the spacing in said one sweepcoordinate of the landing points of said electron beams on the portionsof said screen illuminated at an intermediate value of deflection ofsaid beams in said one sweep coordinate, said dimension being' less thanthe spacing in said one sweep coordinate of said beam landing points atthe minimum value of deflection of said beams and being more than thespacing of said beam landing points at the maximum value of deflectionof said beams in said one sweep coordinate.

3. In a cathode ray tube, an image screen having two phosphors, eachphosphor group consisting of respec-- tive phosphor areas adapted to beilluminated by electrons from different rsepective electron guns, thephosphor areas in each group having a dimension and a center to centerspacing in at least one screen coordinate which decreases progressivelyfrom the centervto the edge of the screen.

said dimension being.

screen than in the peripheral portion of the screen, said screencomprising a plurality of groups of phosphors,

arrayed in two coordinates, each phosphor group consisting of respectivephosphor areas adapted to be illuminated by electrons from diflerentrespective guns, the phosphor areas in each group having an equaldimension in at least one screen coordinate,- said dimension beingapproximately equal to the spacing in said one screen coordinate of thelanding points of electron beams from said respective electron guns onthe purtions of said screen at the extremes of said one coordinate.

5. In a cathode ray tube, a screen, a plurality of electron gunsarranged to illuminate the screen with electron beams directed at aplane of deflection between the guns and screen, an electron permeablemask between said guns and screen adapted to have a potential diiferencewith respect to said screen such as to form an accelerating electricfield therebetween, deflection means for sweeping each beam across thescreen in two perpendicular coordinates by angular deflection from acenter of deflection at the intersection of the beam with the plane ofdeflection, said centers of deflection of adjacent beams having aspacing 8,, a plurality of groups of phosphors on said screen, eachgroup consisting of respective phosphor areas of sub-elemental imagesize in at least one sweep coordinate and adapted to be illuminated bythe respective electron beams, the elemental phosphor areas of eachphosphor group having a uniform dimension and a substantially n- "'terto center spacing in said one sweep coordinate, .9,

equal to dcs where D is the distance from the center of deflection tothe mask 10 d is the mask to screen spacing, g is V,,-V m V =screenpotential, V,,',=mask potential, and S is less than 8,. r

6. A cathode ray tube as defined in claim '5 wherein 8:75 to 95% of 8,.

References Cited in the file of this patent UNITED STATES PATENTS2,435,889 Kerridge Feb. 10, 1948 2,446,915 Filmer Aug. 10, 19482,568,448 Hansen Sept. 18, 1951 2,722,623 La-w Nov. 1, 1955 2,728,024Ramberg Dec. 20, 1955 2,743,391 Hoagland Apr. 24, 1956 2,747,134 AllwineMay 22, 1956 2,755,402 Morrell July 17, 1956 2,757,112 Hoyt July 3 1,1956 2,793,317 Lawrence May 21, 1957 2,795,720 Epstein June 11, 1957FOREIGN PATENTS 866,065

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